The invention relates to a method for control a printing process, in particular with digital printing units.
Preparing necessary information, i.e., so-called printing data, that is already suited for the predetermined mechanical printing conditions and the predetermined printing method and correspondingly establishing the area coverages and colors in a digital database during a prepress stage, i.e., during a production stage upstream from the actual mechanical printing process, is generally known. The printing process is then carried out based on these data in a printing press such as, for example, an ink jet printing unit or a laser printing unit, and these digital printing data are no longer modified during printing production. In the case of offset printing methods or other printing methods with physically fixed printing forms, printing plates or other printing forms are fabricated based on these data, which are then used to generate several thousand printed copies for example. In multicolor printing that is typically used nowadays, color image data designed and prepared during the creative prepress stage are broken down into color separations. These separations are then printed during the printing process with the printing inks available in the case of the relevant printing method. The color impression of the different color tones contained in the color image data is produced when the color separations are printed together in the available printing inks. The color separations contain for every pixel the relative coverages of the respectively used printing inks, as a rule cyan, magenta, yellow and black (CMYK). Breaking down the color image data of the job usage data into the individual color separations normally takes place using the so-called RIP process. RIP is the common abbreviation for “raster image processor”. In this process, the page objects (images, text, graphic elements, etc.) contained in the job usage file are converted into the pixel-oriented image suitable for the relevant digital printing method. In doing so, the breakdown into the individual color separations for the printing inks used in the printing method is carried out, wherein a one-color color separation file is generated for each color separation of one of these printing inks in the pixel-oriented image data format suitable for the printing method (e.g., Tiff, wherein TIFF is the abbreviation for “tagged image file format”, a halftone image file format).
Furthermore, systems are known which ascertain the color reproduction in the printing process using corresponding measuring techniques (inline, online, offline) and control or regulate using automatic or manual feedback. However, in the process only printing-process-specific parameters for the printing press are controlled or regulated, and the digital printing data established during the prepress stage remain unchanged.
In electrographics, for example the charging potential of the photoconductor, the depth of discharge from the exposure unit, the direct-current potential in a developer system or the toner concentration may be adjusted as printing-process-specific parameters, in order to influence the printed result or the print quality.
In the case of inkjet printing units, the drop size among other things may be modulated as printing-process-specific parameters.
It turns out in practice according to the prior art that the possibilities of influencing the print quality, especially the color reproduction solely by changing the printing-process-specific parameters on the printing press are very limited.
For example, many of the common control and regulation parameters for influencing color reproduction are not locally controlled, but have an effect on the entire printed area. Therefore, it is not possible to influence individual elements in the subject in terms of color with the available technology.
In the case of inkjet printing units, the possibilities for control and regulation according to the prior art are basically restricted, because the adjustable drop volumes are normally inherent in the system, even though some manufacturers offer a certain number of different drop sizes. A finely graduated control can only be realized with difficulty or not at all with the technologies available in this environment. In addition, the color reproduction cannot be influenced selectively by color with the known technology.
The object of the invention is creating an improved method for controlling a printing process.
The core of the method according to the invention must be seen as effecting control or exerting influence by correcting or changing the digital data that are generated during the prepress stage instead of the approach that is common nowadays, i.e., influencing print quality using the available control and regulations parameters of the printing unit.
In this case, measured values, particularly spectral values, on the printed product describing the actual result of the print run in terms of color are ascertained and compared with predetermined desired values. Corresponding color corrections are derived from the deviations between the actual and desired values that are yielded. Using the calculated correction, the printing data from the prepress stage are modified. A dedicated color calculator may be used for this. In the same way, already existing hardware and software may be used for this correction.
The control method according to the invention may be advantageously integrated in the entire printing process in different variants.
Thus, for example the color calculator can be integrated into the printing press. It is also advantageously possible to carry out the correction of the printing data outside of the printing press during the data preparation upstream from the printing. The correction may also be carried out on the basis of individually printed pages or individual page objects or on the basis of an imposed digital printing form.
Ascertaining the measured data may be carried out in this case either inline (i.e., measuring in the printing unit during the ongoing printing process) or online (i.e., the measuring device is connected in terms of its data with the control and regulation system, however, the freshly printed print product is removed from the ongoing printing process for the measuring process and measured outside the printing press).
Camera-based measuring system among others may be used for the measurement. Predetermined measuring elements, e.g., predefined measuring fields of an integrated print control strip, may be measured among other things. In the same way, specified regions of the printed area or else the entire printed area may also be ascertained. This type of measurement in non-predetermined measuring fields but in any desired regions of the printed product is also referred to as “measurement in the image”. In the case of the last-mentioned embodiment, there is the advantageous possibility of concentrating especially on the colors in the printed product that are critical for the job and to reproduce these with the best possible approximation via a suitable correction of the job usage data.
The method according to the invention makes a much more precise adjustment of the color reproduction of the printing data and is especially advantageous for an individual treatment of the individual regions of the printed area.
At the same time, it is possible with the method according to the invention to considerably reduce the control intervals in an advantageous manner and carry out an adjustment of the color reproduction almost in real time. In addition, the method according to the invention advantageously makes the selective adjustment of individual color regions possible.
The group of printing methods in which the printed image is transmitted directly from a computer to a printing press/printing unit without a static printing form being used should be understood as digital printing with respect to the invention. These are preferably inkjet printing units or electrophotographic printing systems such as a laser printer, which are preferably designed for high print run numbers. Of course, other digital printing units may also be provided.
Advantageous embodiments and expedient further developments of the higher-level measures are disclosed in the dependent claims and are given in greater detail in the following description of the example based on the schematic drawing.
The drawings described below show the following.